To simplify the description, a synchronous tapping function, which is a representative function for position synchronization between a spindle and a subordinate axis, is taken up as an example in the following description. FIGS. 14, 15 and 16 show a spindle driving apparatus and a Z-axis servo driving apparatus, and in FIGS. 14, 15 and 16 an alternately long and short dashed line indicates a digital value provided by software.
In FIG. 14, reference numeral 1 indicates a numerical value controlling apparatus (called an controlling apparatus hereinafter) which issues a speed instruction .omega.rs*, and position instructions .theta.rs* and .theta.rz*. Reference numerals 2, 2a indicate a spindle driving apparatus and a Z-axis servo driving apparatus (called Z-axis driving apparatus hereinafter) each connected to the controlling apparatus 1 respectively. Reference numerals 3, 3a indicate a spindle motor connected to the spindle driving apparatus 2 and a Z-axis motor connected to the Z-axis driving apparatus 2a respectively. The reference numerals 4, 4a indicate a spindle position detector which is directly connected to a revolving shaft of the spindle motor 3 and generates, for instance, output of 256 waves per rotation, and a Z-axis position detector which is directly connected to a revolving shaft of the Z-axis motor 3a and generates, for instance, 2500 pulses per rotation respectively. Reference numerals 17, 17a indicate a spindle position detecting circuit which receives an output signal from the spindle position detector 4 and generates a position detection signal .theta.rs and a Z-axis position detecting circuit which receives an output signal from the Z-axis position detector 4a and generates a position detection signal .theta.rz. Reference numeral 5 indicates a spindle in the machine tool with the revolving shaft driven by the spindle motor 3 and the vertical shaft (Z-axis) controlled by the Z-axis motor 3a. Reference numeral 5a indicates a Z-axis table which slides the spindle 5 in the vertical direction. Reference numerals 6, 6a indicate a spindle gear which connect the spindle motor 3 to the spindle 5 and a Z-axis gear which connects the Z-axis motor 3a to the Z-axis table 5a. Reference numeral 7 indicates a tool (tapper) for tapping, and reference numeral 8 indicates a differentiator which receives and differentiates position detection signal .theta.rs and position detection signal .theta.rz, and issues speed detection signals .omega.rs and .omega.rz.
In FIG. 15, reference numerals 10, 10a indicate comparators which compare position instructions .theta.rs*, .theta.rz* to position detecting signal .theta.rs, .theta.rz and output position deviation signal .DELTA..theta.rs, .DELTA..theta.rz. Reference numerals 11, 11a indicate position loop gain circuits which are connected to the comparators 10, 10a respectively and amplify the position deviation signals .DELTA..theta.rs and .DELTA..theta.rz according to position loop gains K.sub.PS, K.sub.PZ, respectively. Reference numeral 12 indicates a mode select switch having a contact A to which the speed instruction .omega.rs* is entered as well as a contact B to which output from the position loop gain circuit 11 is entered and, selecting contact A in a speed control mode for controlling revolving speed of the spindle 5, and selecting contact B in a position control mode for controlling rotational position of the spindle 5. Reference numeral 13 indicates a comparator which compares output .omega.rs* to speed detection signal .omega.rs and provides a speed deviation signal .DELTA..omega.rs as output. Reference numeral 13a indicates a comparator which compares output .omega.rz* from position loop gain circuit 11a to the speed detection signal .omega.rz and provides speed deviation signal .notident..omega.rz. Reference numerals 14, 14a indicate speed loop gain circuits which amplify the speed deviation signals .DELTA..omega.rs, .DELTA..omega.rz according to speed loop gains K.sub.vs, K.sub.vz respectively and provide current instructions Is'*, Iz'*. Reference numerals 15, 15a indicate current limiter circuits which limit output Is'*, Iz'* from the speed loop gain circuits 14, 14a respectively to a current value corresponding to the output characteristics of a motor and provide current instructions Is*, Iz* as output respectively. Reference numerals 16, 16a indicate power convertor circuits which supply outputs Is*, Iz* from the power limiter circuits 15, 15a to motors 3, 3a respectively.
In the description below, the operation of the spindle driving apparatus 2 and the Z-axis driving apparatus 2a of a machine tool having a synchronous operation function based on the conventional art is divided into (1) an operation in normal spindle operation mode, and (2) an operation in synchronous tap running mode.
(1) Normal spindle operation mode
In the case of normal spindle operation mode in which synchronous tapping is not carried out, mode select switch 12 in the spindle driving apparatus 2 is set to contact A. The speed instruction .omega.rs* corresponding to a target number of rotations for the spindle 5 is issued from the controlling apparatus 1. Accordingly, the spindle driving apparatus 2 provide controls to cause a speed (.omega.rs) of the spindle motor 3 to follow the speed instruction .omega.rs*. Namely, the speed instruction .omega.rs* and the speed detection signal .omega.rs are compared by the comparator 13, the speed deviation signal .DELTA..omega.rs is issued from the comparator 13, and the speed deviation signal .DELTA..omega.rs is amplified and provided as the current instruction Is'* in the speed loop gain circuit 14 and then converted to power for driving the spindle motor 3 in a power convertor circuit 16. With this operation, the spindle motor 3 is controlled so that it follows the speed instruction .omega.rs*.
Also in the normal spindle operation mode, the Z-axis driving apparatus 2a works independently from the spindle driving apparatus 2. In this case, the position instruction .theta.rz* and the position detection signal .theta.rz are compared by the comparator 10a. The position deviation signal .DELTA..theta.rz is issued from the comparator 10a and amplified in the position loop gain circuit 11a and then entered into the comparator 13a. The subsequent operations are the same as those of the spindle driving apparatus 2 in the normal spindle operation mode as described above, while the Z-axis table 5a is controlled so that it follows the position instruction .theta.rz* via the Z-axis motor 3a.
(2) synchronous tapping operation mode
In the case of a synchronous tapping operation mode, a rotational position of a spindle and a position of a Z-axis are synchronized. Specifically, an instruction given to the spindle driving apparatus 2 is switched to the position instruction .theta.rs* by the controlling apparatus 1. The spindle driving apparatus 2 detects this condition and switches the mode select switch 12 to contact B. The position instruction .theta.rs* and the position detection signal. .theta.rs are compared by the comparator 10, the position deviation signal .DELTA..theta.rs is issued from the comparator 10, and the position deviation signal .DELTA..theta.rs is amplified by the position loop gain circuit 11 and entered into the comparator 13 as a speed instruction. The subsequent operations are the same as those in the normal spindle operation mode as described above. Consequently, and the spindle 5 is controlled via the spindle motor 3 so that it follows the position instruction .theta.rs*.
Also in the case of a synchronous tapping operation mode, the Z-axis driving apparatus 2a receives a Z-axis position instruction .theta.rz*, which is in synchronous relation with the position instruction .theta.rs* to the spindle driving apparatus 2 described above, from the instruction apparatus 1. Also, the Z- axis table 5a is controlled via the Z-axis motor 3a that it follows the position instruction .theta.rz*.
FIG. 16 is a block diagram showing configuration modified from that shown in FIG. 15. In FIG. 16, a torque constant circuit 18, 18a multiplies the output from the speed loop gain circuit 14, 14a with torque constants K.sub.tS, K.sub.tZ and issues the products as torque instructions T.sub.LS *, T.sub.LZ * respectively. Also, a comparator 19, 19a compares the torque instructions T.sub.LS *, T.sub.LZ * to external disturbance torque T.sub.LS, T.sub.LZ, respectively. Finally, a division circuit 20, 20a carries out subtraction with motor inertia J.sub.S, J.sub.Z, and an integrator 21 executes integration.
In the synchronous tapping operation as described above, a response to a speed loop in spindle control system is generally lower than that the Z-axis control system, for the reasons (a) to (c) as described below.
(a) Firstly, the larger a value of a motor (torque/inertia) is, the higher a response to a speed loop is, but the aforesaid value in the Z-axis motor using a synchronous motor therein is substantially larger as compared to that in the spindle motor using a conductive motor therein, and a difference in response exists between each motor unit. Generally in a motor unit, response of the Z-axis is about 5 to 10 times higher than that of the spindle.
(b) Secondly, a ratio of inertia of a motor itself vs load inertia at the side of a machine driven by the motor can be suppressed to below about 2 times in a Z-axis, while that in a spindle is in a range from 1 to 5 times (L gear: 1 to 2 times, M gear: 2 to 3 times, H gear: 4 to 5 times), and a speed loop gain in a spindle, especially an M gear and a H gear, becomes relatively lower.
(c) Thirdly, even if the load inertia described above becomes larger, the responsibility does not drop by increasing the speed loop gain in proportion to increase of the load inertia described above. However, as a larger backlash generally exists in a spindle gear, if a gain is raised too much, vibration is generated due to instability. For this reason, even if load inertia becomes larger, the speed loop gain can not be raised.
This difference in speed loop response between a spindle control system and a Z-axis control system may sometimes cause a position error between the two in operations for acceleration or deceleration or in fluctuation due to load external disturbance, each of which is a transitional state in a synchronous tapping operation, which in turns give influence over a thread cutting in the synchronous tapping operation. In the conventional art, as described above, a position differences of the two becomes larger in an H gear. This causes a problem concerning a thread cutting precision of a tap, so that synchronous tapping is performed only in an L gear and an M gear, in each of which has a relatively small load inertia.
Technological documents relating to the present invention include the Japanese Patent Laid Open Publication No. SHO 59-191606 disclosing "a synchronous operation system", the Japanese Patent Laid Open Publication No. SHO 64-16285 (HEI 1-16285) disclosing "an invertor controlling apparatus", the Japanese Patent Laid Open Publication No. SHO 64-27808 (HEI 1-27808) disclosing "a numerical controlling apparatus", and the Japanese Patent Laid Open No. SHO 63-89904 disclosing "a numerical controlling apparatus".
In an instruction apparatus for a machine tool having a synchronous tapping function based on the conventional art as described above, a similar position instruction is issued to a Z-axis controlling apparatus in both the normal spindle operation mode and the synchronous tapping operation mode. In the normal spindle operation mode in which position synchronization between a spindle and a Z-axis is not necessary, a difference in speed response between two does not cause any specific problem, but in synchronous tapping operation mode in which position synchronization is required as described above, a difference in speed response between the two gives an influence over the thread cutting precision of a tap.
In order to raise the precision, generally a position loop gain for the two is equalized to make the position orbit for the two identical, as a minimum requirement. In this case, however, a relative position error between the two can occur due to a difference in speed response. Also, since, the position loop gain for the Z-axis must be set to a value that is a little lower than the actual the spindle side, the Z-axis position becomes vulnerable to fluctuation due to external disturbance in proportion to the difference. Furthermore, if the tapping time constant is made larger to reduce relative position error between the spindle and the Z-axis position by reducing the transmission ratio for acceleration and deceleration, then the cycle time becomes longer, which degrades productivity.
Also in the conventional art, as described above, synchronous taping is not executed with an H gear, but with an L gear and an M gear for improving precision, and for minimizing a difference in speed response between a spindle and a Z-axis. Nonetheless, a response delay to the Z-axis exists. Furthermore, as the maximum speed of the spindle becomes lower when synchronous tapping is executed with an L gear and an M gear as compared to that when executed with an H gear, a high speed tapping cycle can not be achieved which also reduces productivity.
The aforesaid problems are concretely described hereinafter with reference to FIG. 13. FIG. 13, waveform (a) shows speed of a spindle and the Z-axis versus time. As shown in FIG. 13(a), the speed line of spindle does not coincide with the speed line of Z-axis, because the spindle is a rotating axis and the Z-axis is a linear axis. Accordingly, they are significant differences in relative position and a wide range. Namely, in FIG. 13, waveform (c), when executing synchronous tapping, a speed loop response in a spindle control system is generally lower as compared to that in a Z-axis control system. For instance, because the load AD 2 is large and a motor inertia is larger. Accordingly, even if a position loop gain is identical, a difference in the speed loop response or a position loop gain in the Z-axis is set to a somewhat lower value for the spindle. Accordingly, a large relative position difference may occur when, for instance, the speed rapidly changes (t4, t6, t7, t9, t10 etc.) or when a load due to external disturbance rapidly changes (t5, tS, t11, etc.).